The invention relates to a magnetic trip device with a wide tripping threshold setting range, comprising a fixed magnetic circuit bearing an excitation coil and having a first and a second polar part located one in the extension of the other and separated by a fixed air-gap and a moving core assembly, slidingly mounted inside said fixed magnetic circuit and having a first core assembly part with a polar surface defining a first variable air-gap with said first polar part, the value of which varies with the movement of the moving core assembly.
A magnetic trip device of the kind mentioned is generally associated with a current breaking device, notably a circuit breaker protecting lines or equipment against short-circuit currents. The tripping threshold must be suited to the installation protected, and trip devices advantageously include adjustment of this threshold. State-of-the-art setting adjustments act either on the return force of the trip device moving assembly or on the length of the air-gap, but the adjustment possibilities are limited, and these adjustments are not linear. In some applications, the setting range must be large, as the threshold can vary from 1 to 5 or even more, in particular when protecting an electric motor is involved. State-of-the-art adjustment devices are unable to provide linearity over such a wide setting range and do not meet the present requirements.
The object of the invention is to achieve a magnetic trip device with a wide setting range appreciably linear over the whole setting range, while preserving the simplicity indispensable for satisfactory operation.